Live attenuated virus vaccines have been successfully used for centuries for the prevention of viral infectious disease. In the past, live attenuated vaccine viruses were either isolated in the field or generated through elaborate passaging regimens in a diversity of hosts. The possibility to manipulate the genetic makeup of viruses at will has, for the first time, enabled us to generate live attenuated viruses through genetic engineering. We are pursuing an ambitious program to generate live attenuated poliovirus vaccine vectors expressing immunogenic material derived of HIV. Infection with live attenuated poliovirus vaccines is known to induce potent immune responses upon replication in the gastrointestinal tract of immunized children. By engineering poliovirus to express HIV genetic material, we aim to generate such responses against heterologous retroviral gene products for prophylaxis against HIV infection. All attempts to utilize the advantageous properties of live attenuated polioviruses for the generation of HIV immunization vehicles have failed because of the intrinsic genetic instability of these viral agents. Insertion of foreign genetic material is likely to interfere with efficient replication kinetics of poliovirus and, hence, triggers genetic adaptation events that lead to the elimination of heterologous inserts. Consequently, poliovirus-based expression vectors generated so far, were characterized by rapid deletion of HIV-related genetic material, severely limiting their applicability. We have created a new strategy to produce poliovirus-based expression vectors with indefinite genetic stability and highly efficient foreign insert expression. This strategy exploits genetic plasticity of an indispensable genetic element involved in poliovirus translational control. We have achieved to replace regulatory sequences within the viral internal ribosomal entry site with foreign sequences encoding for HIV-related gene products. Since, in our vector, foreign inserts were designed to replace viral sequences with beneficial regulatory functions during viral replication, they were retained indefinitely by replicating expression vectors. We will pursue to identify favorable structural characteristics leading to insert retention and efficient expression in our vectors to prepare them for HIV immunization studies in non-human primates.